Pine Island Glacier, located in West Antarctica, is showing
signs of thinning making it more susceptible to climatic and ocean variability
than at first thought. Scientists led by the British Antarctic Survey have
discovered large fluctuations in the ocean heat manifesting itself in the
melting of the ice shelf into which the glacier flows. Between 2010 and 2012 the
ice shelf into which the ice stream flows has decreased by 50%, most likely due
to La Ninã, suggesting a complex interplay between geological,
oceanographic and climatic processes.

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Driven by acceleration in its flow, Pine Island Glacier has thinned continuously. The acceleration is thought to be caused by thinning of the floating ice shelf created as the glacier slides into the sea. Understanding the cause and effect between ice shelf thinning and glacial response is essential in assessing the contributions to rising sea levels.

Much of the thinning is due to a deep oceanic inflow of Circumpolar Deep Water (CDW) on the continental shelf neighboring the glacier. This warmer water then makes its way into a cavity beneath the ice shelf melting it from below.

In 2009, a higher CDW volume and temperature in Pine Island Bay contributed to an increase in ice shelf melting compared to the last time measurements taken in 1994. But observations made in January 2012, and reported now in Science, show ocean melting of the glacier was the lowest ever recorded. The top of the thermocline (the layer separating cold surface water and warm deep waters) was found to be about 250 meters deeper compared with any other year for which measurements exist.

This lowered thermocline reduces the amount of heat flowing over the ridge. High resolution simulations of the ocean circulation in the ice shelf cavity demonstrate that the ridge blocks the deepest ocean waters from reaching the thickest ice enhancing the shelf’s sensitivity to climate variability.

The temperature fluctuations may be explained by particular climatic conditions. In January 2012 the dramatic cooling of the ocean around the glacier is believed to be due to an increase in easterly winds from the La Ninã event in the tropical Pacific Ocean. Normally the winds flow from the west.

The study stresses the importance of both local geology and climate variability in ocean melting in this region.